Insects that reproduce sexually must locate a suitable mate, and many species have evolved efficient communication mechanisms to find each other. The number of reproductively viable individuals in a population can be an important constraint in the growth of populations. One factor that can affect insect fecundity is the age of mating adults, as fecundity tends to decline with age. Field observations collected annually on Lymantria dispar (L.) from 2001 to 2007 and 2009 consistently revealed a small proportion of egg masses (generally <10% in each year) in which >0 but <5% of eggs were fertilized in an egg mass consisting of ≈200–500 eggs. In these studies, male age was unknown but female age was fixed at <24 h, which, according to previous studies on the effect of female L. dispar age on reproductive success, should have been optimal for fertilization. In this article, we analyzed field data (2001–2007 and 2009) to explore patterns in the occurrence of low-fertilized egg masses.We supplemented these data with laboratory experiments that examined the interacting role of male and female age, and multiple male matings. We observed that increases in male and female age reduce the rate of fertilization, which is furthermore reduced, as males mate multiple times as they age. This article highlights the importance of both female and male age at the time of mating in an invading species, with ramifications to low-density populations in this and other sexually reproducing insect species.